Solenoid-operated proportional flow control valve

ABSTRACT

An electromagnetic proportional flow rate control valve capable of performing precise flow rate control in a low flow rate side of an operating region is provided. In an electromagnetic proportional flow rate control valve  32,  in which a valve member  2  is driven in a valve closing direction in accordance with the increase in a driving current I of a solenoid coil  15,  a solenoid driving force F is set such that it decreases accompanying the displacement of the valve member  2  in the valve closing direction with respect to an identical driving current in the solenoid coil  15.  Thus, an amount of change in an area of a valve opening with respect to unit changes in the driving current I becomes smaller in a region at which the stroke is small compared to a case of having flat characteristics, whereby the flow rate control precision in the low flow rate region can be improved.

TECHNICAL FIELD

This invention relates to an improvement of an electromagneticproportional flow rate control valve used for power steering apparatusesof automobiles, industrial machines, and the like.

BACKGROUND ART

The device disclosed in Japanese Patent Application Laid-open No.2001-163233, for example, is a conventional electromagnetic proportionalflow rate control valve used for power steering apparatuses.

In the electromagnetic flow rate control valve described above, as shownby dashed lines in FIG. 3, a driving force of a solenoid for driving avalve member is set to have flat characteristics so that the solenoiddriving force becomes uniform over nearly an entire region of a valvestroke with respect to an identical driving current.

In general, as shown by double dashed lines in FIG. 3, the solenoiddriving force sharply increases on an adsorption side in accordance witha valve stroke with respect to an identical driving current. However,since control characteristics of the valve become complex in this case,the solenoid driving force is set so as to have flat characteristics,thereby being capable of making control of a valve-opening degreeeasier.

However, even if the solenoid driving force is set to have flatcharacteristics, between the driving current that flows in a solenoidcoil and the solenoid driving force, there is such a relationship thatthe solenoid driving force becomes relatively large with respect to achange in a unit current value in a region where the current valuebecomes large. This is because the solenoid driving force isproportional to the square of the current value. The extent of change inthe solenoid driving force therefore gradually becomes larger withrespect to the same amount of change in the current, as shown in FIG. 3.

As a result, as shown by a dashed line in FIG. 4, the extent of thechange in the solenoid driving force with respect to an identical amountof change in current becomes larger in the region where the value of thesolenoid driving current is large, compared with a region where thecurrent value is small.

The degree of opening of the valve generally becomes a maximum in astate at which the driving current is zero in the electromagneticproportional flow rate control valve, and the degree of opening thereofdecreases along with the increase in the driving current. That is, thecontrol amount is set so as to decrease in accordance with the increasein the driving current.

The flow rate to be controlled, therefore, had a tendency to changesharply with respect to a slight change in the driving current in theregion where the opening degree of the electromagnetic proportionalcontrol valve is small. A problem consequently developed in thataccurate flow rate control becomes difficult in a low flow rate regionwhere the flow rate to be controlled is small.

An object of this invention is to provide an electromagneticproportional flow rate control valve capable of performing flow ratecontrol with high precision in the low flow rate control region.

DISCLOSURE OF THE INVENTION

According to the present invention, a solenoid driving force withrespect to a valve stroke is given such characteristics that there is adrop off on an adsorption side for an identical driving current, thatis, a valve opening degree is made smaller. Thus, compared with a caseof flat characteristics described above, an amount of change in an areaof the valve opening is small with respect to unit changes in thedriving current in a region where the stroke of the valve member isshort, with the result that flow rate control precision is increased ina low flow rate control region.

Further, according to the present invention, the electromagneticproportional flow rate control valve is structured such that theproportion of reduction in the area of the opening of a variable orificegradually becomes smaller with respect to the valve stroke as the valvemember approaches a valve seat. Thus, the flow rate control precision isincreased in the low flow rate control region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a hydraulic circuit diagram of a power steering apparatusaccording to this invention.

FIG. 2 is a cross sectional view showing an electromagnetic proportionalflow rate control valve according to the present invention.

FIG. 3 is a characteristic diagram showing a relationship between avalve stroke and a solenoid driving force in the electromagneticproportional flow rate control valve according to the present invention.

FIG. 4 is a characteristic diagram showing a relationship between adriving current and the solenoid driving force according to the presentinvention.

FIG. 5 is a characteristic diagram showing a relationship between adriving current and a control flow rate according to the presentinvention.

FIG. 6 is a cross sectional view showing an electromagnetic proportionalflow rate control valve in accordance with another embodiment of thisinvention.

FIG. 7 is a characteristic diagram showing a relationship between adriving current and a solenoid driving force in accordance with anotherembodiment of this invention.

FIG. 8 is a characteristic diagram showing a relationship between thedriving current and a control flow rate in accordance with anotherembodiment of this invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of this invention will be described hereinbelow withreference to the drawings.

FIG. 1 is a hydraulic circuit diagram of a power steering apparatus forautomobiles.

Reference numeral 31 denotes a pump. Hydraulic oil discharged from thepump 31 is supplied to a power steering system 36 for assisting steeringoperations in an automobile. An electromagnetic proportional flow ratecontrol valve 32 is provided in order to control the flow rate of thehydraulic oil supplied to the power steering system 36.

The flow rate of the hydraulic oil controlled by the electromagneticproportional flow rate control valve 32 is proportional to the valveopening degree of the electromagnetic proportional flow rate controlvalve 32, and to a pressure difference between an upstream anddownstream of the valve. Accordingly, if the pressure difference isconstant, the flow rate is changed depending upon solely the openingdegree of the valve 32. Accordingly, in order to obtain a flow rate thatis proportional to the opening degree of the electromagneticproportional flow rate control valve 32, a pressure compensating valve33 is provided upstream of the electromagnetic proportional flow ratecontrol valve 32. The pressure compensating valve 33 serves tosubstantially maintain a certain pressure difference ΔP (=P₁−P₂) in thehydraulic circuit between the upstream and downstream of theelectromagnetic proportional flow rate control valve 32.

If the pressure difference becomes larger than a set value, the pressurecompensating valve 33 releases a portion of the hydraulic oil from theupstream of the electromagnetic proportional flow rate control valve 32to a reservoir 39, thereby lowering the pressure on the upstream sidethereof to maintain the pressure difference into the set value. Contraryto this, if the pressure difference becomes smaller than the set value,the pressure compensating valve 33 reduces the amount of hydraulic oilto be released from the upstream of the electromagnetic proportionalflow rate control valve 32 to the reservoir 39, thereby increasing thepressure on the upstream side thereof to maintain the pressuredifference into the set value.

Reference numeral 34 denotes an orifice, and reference numeral 35denotes a high pressure relief valve. The relief valve 35 determines amaximum pressure of the hydraulic oil to be supplied to the powersteering system 36, and therefore functions as a safety valve. Therelief valve 35 opens, if the oil pressure is equal to or greater than aset pressure, to release the hydraulic oil from the upstream of theelectromagnetic proportional flow rate control valve 32 to the reservoir39. Further, the orifice 34 suppresses fluctuations in pressure;communicates with the pressure compensating valve 33; and contributes tooperational stability.

In this power steering apparatus, during non-steering state in which asteering wheel is kept in the neutral position, a load pressure P₂ ofthe power steering system 36 is reduced, and the required flow rate ofthe hydraulic oil is also small. Consequently, the electromagneticproportional flow rate control valve 32 maintains a minimum openingdegree thereof. Only the minimum flow rate determined by the minimumopening degree thereof is supplied to the power steering system 36, andthe control flow rate to be supplied to the power steering system 36 islessened, thereby reducing the energy loss.

In contrast, the load pressure P₂ of the power steering system 36becomes high during the steering operation, and the required hydraulicoil flow rate also becomes large. Accordingly, the opening degree of theelectromagnetic proportional flow rate control valve 32 is alsocontrolled so as to become large. With this, a flow rate Qc, which iscontrolled in accordance with the opening degree of the electromagneticproportional flow rate control valve 32, is supplied to the powersteering system 36, and a necessary power assisting force may beimparted thereto.

The specific structure of the electromagnetic proportional flow ratecontrol valve 32 is shown in FIG. 2.

The electromagnetic proportional flow rate control valve 32 is providedwith a cylindrical valve body 1 attached to a portion of a housing 8 ofthe pump 31 for supplying hydraulic oil, and a valve member 2 sidablyinserted into an inner circumferential surface 5 of the valve body 1.

An upstream port 21 that is communicated to a discharge side of the pump31; a valve seat 16 that forms a variable orifice 22 in association withthe valve member 2; and a downstream port 23 that is communicated to aload side are formed in the valve body 1. Hydraulic fluid dischargedfrom the pump 31 is allowed to flow to the load side through theupstream port 21, the variable orifice 22 (the valve seat 16), and thedownstream port 23, as indicated by an arrow in the figure. That is, thehydraulic fluid flows to the power steering system 36.

The cylindrical valve member 2 is slidably supported by a pair ofbearings 9 a and 9 b within the valve body 1 and a sleeve 10 coaxiallycoupled with the valve body 1.

A conical valve portion 2 a is formed at a tip of the valve member 2,and the valve portion 2 a is inserted into the valve seat 16. An area Avof the opening of the variable orifice 22 formed between the valveportion 2 a and the valve seat 16 gradually becomes larger, accompanyingdisplacement of the valve member 2 in a direction away from the valveseat 16 in the figure (a rightward direction in FIG. 2).

A spring 13 is provided for urging the valve member 2 in a directionaway from the valve seat 16, i.e., a valve opening direction, and aspring 14 is provided for urging the valve member 2 in the oppositedirection, i.e., a valve closing direction.

A plunger 6 is fixed to the middle of the valve member 2 as a movablecore, and a solenoid coil 15 for driving the plunger 6 is disposed onthe outside of the sleeve 10. The plunger 6 drives the valve member 2 inthe valve closing direction by a solenoid driving force F of thesolenoid coil 15. That is, the valve member 2 is displaced in a leftwarddirection in FIG. 2 against the spring force of the spring 13, inaccordance with the increase in a driving current I flowing in thesolenoid coil 15.

Specifically, the solenoid driving force F generated by the solenoidcoil 15 acts on the valve member 2 in the valve closing direction.Opposing this, a differential force between the springs 13 and 14 (sincethe spring 13 works in the valve opening direction, and the spring 14works in the valve closing direction, this differential force is adifference between the two spring forces, although the acting force ofthe spring 13 is set larger so that the overall force acts in the valveopening direction), a force generated due to the pressure difference ΔPbetween the pressures on the upstream and the downstream of the variableorifice 22, and a fluidic force that develops at the variable orifice 22act on the valve member 2 in the valve opening direction. The valvemember 2 moves to a position at which those forces are in balance. Thus,the area of the variable orifice 22, that is, the area Av of the openingof the valve member 2 is determined.

The state shown in FIG. 2 is one in which: the driving current withrespect to the solenoid coil 15 is large; the valve member 2 isdisplaced to the maximum and can not be displaced any further, becausethe plunger 6 is brought into contact with an inner circumferential stepportion 3 of the valve body 1, which is an adsorption portion, through aring 4; and the area Av of the valve opening is the minimum. Note thatas discussed later, in this state. valve stroke is zero (stroke S=0), tothe contrary the driving current I with respect to the solenoid valve 15is zero and the valve is maintained at a maximum opening degree thereofby the spring force, is defined as a maximum stroke state in thisspecification.

In the present invention, the relationship between the solenoid drivingforce F of the solenoid coil 15 acting on the valve member 2 and thestroke S of the valve member 2 is set so as to be the one shown in FIG.3.

FIG. 3 is a characteristic diagram showing a relationship between thesolenoid driving force F of the solenoid coil 15 and the stroke S of thevalve member 2 in accordance with changes in a driving current I.

Note that the left edge position shown in FIG. 3 is determined as aposition at which the stroke S is zero, and the stroke amount becomeslarger toward the right side.

If the driving current I provided to the solenoid coil 15 is small, thesolenoid driving force F is also small. The solenoid driving forcebecomes relatively larger accompanying the increase in the current.

On the other hand, the relationship between the stroke position and thesolenoid driving force F approaches substantially flat characteristicsfor an identical driving current I. However, the solenoid driving forceF is set so as to gradually become smaller as the amount of stroke ofthe valve member 2 approaches zero, that is, as the valve portion 2 aapproaches the valve seat 16. This kind of setting can be achieved byregulating the shape of a suction portion, as discussed later.

The stroke position of the valve member 2 is determined based on mainlythe differential force between the springs 13 and 14 with respect to agiven solenoid driving force F, as described above. The differentialforce between the springs 13 and 14 changes in accordance with theamount of stroke of the valve member 2. The differential force is thesmallest in an initial state when current does not flow in the solenoidcoil 15 (when the stroke is the largest), and is the largest when thevalve stroke is zero (S=0) and the amount of deformation is the largest.

It is therefore necessary to increase the driving current of thesolenoid coil 15 so that the solenoid driving force F becomes largeragainst the spring differential force, in order to displace the valvemember 2 from the initial state toward the stroke S=0 and to make thearea Av of the opening small.

A adsorbing force of the valve apparatus will generally become larger ifa driving current to the solenoid coil is made large. The suction forcealso changes in accordance with distance from the adsorption portion,that is, in accordance with the valve stroke. If the suction forcechanges in accordance with the valve stroke position, the suction forceof the valve apparatus by an identical current will fluctuate due to thestroke position. This is not preferable for an electromagneticproportional flow rate control valve in terms of control.

Therefore, the shape of the suction portion is being changedconventionally so that the suction force acting on the valve apparatusby the solenoid coil does not change, even if the valve stoke positiondiffers.

The characteristics shown by dashed lines of FIG. 3 show this. For anidentical current, the solenoid driving force F does not change even ifthe stroke S differs, and an identical value may be maintained.

However, the relationship between the solenoid driving force F and thestroke S is set to have flat characteristics over all stroke regions.Therefore, as can be seen from the dashed line of FIG. 4, which showsthe relationship between the driving current and the solenoid drivingforce, the solenoid driving force F changes sharply in a region wherethe driving current I becomes larger, that is, a region where the areaof the valve opening is small, even if the current value only changesslightly.

This is because the solenoid driving force is proportional to the squareof the driving current value, and therefore an extent of change in thesolenoid driving force with respect to unit changes in the current valuebecomes larger in the region where the driving current Is large.

Similarly, regarding the relationship between the driving current I ofthe solenoid coil 15 and the control flow rate Qc, shown in FIG. 5, thearea of the valve opening becomes sharply smaller with respect to thechange in the current, and the control flow rate Qc is also reducedsharply, in the region where the current I becomes larger, as shown bythe dashed line characteristic of FIG. 5.

A large flow rate fluctuation thus develops due to a slight currentfluctuation in the low flow rate region, and the electromagneticproportional flow rate control valve cannot perform the control withprecision.

In contrast, in this invention, the relationship between the stroke Sand the solenoid driving force F for an identical driving current I issubstantially flat in principle, as described above. However, therelation therebetween is set so that the driving force F is graduallyreduced in accordance with the reduction in the stroke S (the reductionin the area Av of the valve opening).

The extent of change in the solenoid driving force F with respect tochanges in an identical driving current therefore becomes small in aregion where the stroke S is small. As a result, the degree of thereduction in the area of the valve opening is suppressed compared to thecase of having the flat characteristics described above. That is, theamount of change in the area of the valve opening with respect to unitchanges in the driving current I becomes smaller in the region where thestroke is small compared to the case of having the flat characteristics.

The relationship between the driving current I and the solenoid drivingforce F is therefore linearly proportional, as shown by thecharacteristics indicated by the solid line in FIG. 4. In particular,there are no sharp fluctuations in the driving force F due to slightcurrent fluctuations in the region where the current value becomeslarger.

Therefore, the relationship between the driving current I and thecontrol flow rate Qc maintains a proportional relationship, and there isno large fluctuation in the flow rate due to a slight change in thecurrent, even in a region where the driving current I becomes larger,the stroke amount approaches zero, and the area Av of the opening of thevariable orifice 22 is small, as shown by the characteristics indicatedby the solid line in FIG. 5.

With this, the flow rate control precision can be made high in the lowflow rate region, and the electromagnetic proportional flow rate controlvalve 32 can accurately control the minute flow rate in accordance withchanges in the driving current I. As a result, control of a fineassisting force of the power steering system 36 becomes possible,thereby obtaining a suitable rigid steering feeling capable of keepingthe steering wheel in the neutral position, which leads to animprovement in the steering feeling.

The relationship between the stroke S and the solenoid driving force Fbecomes substantially flat characteristics with respect to the strokechanges for the identical driving current I, as shown by the solid linein FIG. 3. The driving force F can be made to gradually reduce inaccordance with the reduction in the stroke S by changing the shape ofthe adsorption portion of the valve member 2.

Note that it is conventionally known that the shape of the suctionportion of the valve member may be changed in order to obtain a constantdriving force F over all stroke regions for the identical drivingcurrent I. The relationship between the stroke S and the driving force Fcan be freely changed by the shape of the adsorption portion. However,establishing a relationship in which the driving force F is graduallyreduced in the stroke region where the area Av of the valve openingbecomes small, has not been conventionally performed.

Specifically, in order to establish the relationship described above, inthe present invention, such a structure is taken in which a concaveportion 1 a used for magnetic field regulation, depressed into anannular shape, is formed in a position extending to the innercircumferential step portion 3 in the inner circumferential surface 5,mutually opposed to the valve member 2 of the valve body 1, whichstructures a magnetic circuit with the movable core 6, and an innerdiameter d of the concave portion 1 a used for magnetic field regulationis larger than the inner diameter of the inner circumferential surface 5of the tip of the valve member (left side in the figure).

In addition to this structure, a tapered portion 1 b used for magneticfield regulation may also be formed in an annular edge surface, mutuallyopposing the sleeve 10 of the valve body 1 with a predetermined gaptherebetween. An angle of inclination θ of the tapered portion 1 b maybe suitably set.

Note that, in addition to selectively forming the concave portion 1 aused for magnetic field regulation or the tapered portion 1 b used formagnetic field regulation, both portions may also be formed.

As described above, the characteristics of the solenoid driving force Fshown in FIG. 3 can be obtained by arbitrary setting the values of theinner diameter d of the concave portion la used for magnetic fieldregulation, the tapered portion 1 b used for magnetic field regulation,and the inclination angle θ.

In this case, in order to obtain the characteristics of the control flowrate Qc shown by the solid line of FIG. 5, the valve portion 2 a of thevalve member 2 is formed in a conic shape. As the valve member 2 isdisplaced in the leftward direction in FIG. 2 in association with anincrease in the driving current I, the area Av of the opening of thevariable orifice 22, defined by the valve portion 2 a and the valve seat16, becomes smaller in linear proportion.

From the above results, in the present invention, an effect can beattained, in which the flow rate control precision with respect to thedriving current I can be increased in the region where the openingdegree of the electromagnetic proportional flow rate control valve issmall.

An embodiment of this invention shown in FIG. 6 is explained next.

The valve member 2 a is formed having a substantially spherical shape atthe tip of the valve member 2. The valve portion 2 a is inserted intothe valve seat 16, and the variable orifice 22, defined by the valveportion 2 a and the valve seat 16 is formed. The area Av of the openingof the variable orifice 22 increases or decreases accompanyingdisplacement of the valve member 2 in an axial direction.

The structure is employed such that the valve portion 2 a is formed intoa substantially spherical shape, and the proportion of reduction in thearea of the opening of the variable orifice 22 gradually becomes smallerwith respect to the stroke of the valve member 2 as the valve member 2approaches the valve seat 16.

A cross section of the valve portion 2 a is formed into a substantiallysemi-circular shape, and the valve portion 2 a is made continuous withthe outer circumferential surface 2 b of the valve member 2 without astep.

The characteristic of the relationship between the driving current I ofthe solenoid coil 15 and the solenoid driving force F in this case isone in which the solenoid driving force F increases sharply accompanyingthe increase in current in a region, where the driving current I flowingin the solenoid coil is large, as shown in FIG. 7.

As a countermeasure for this, the valve portion 2 a is formed into asubstantially spherical shape, and a structure is employed, in which theproportion of the reduction in the area Av of the opening of thevariable orifice 22 with respect to the stroke of the valve member 2gradually becomes smaller as the valve member 2 approaches the valveseat 16, thereby precise flow rate control becomes possible on the lowflow rate side of the operating region.

FIG. 8 is a characteristic diagram showing the relationship between thedriving current I of the solenoid coil 15 and the control flow rate Qc.The control flow rate Qc is gently reduced accompanying the increase inthe driving current I flowing in the solenoid coil 15. That is, althoughthe control flow rate Qc is reduced in linear proportion accompanyingthe increase in the driving current I on the high flow rate side of theoperating region, the extent of the reduction in the control flow rateQc gradually drops accompanying the increase in the driving current I onthe low flow rate side of the operating region.

While the flow rate increases and decreases in linear proportion to thedriving current I on the large flow rate side of the operating region,and the flow rate control response characteristic is increased, flowrate fluctuations are finely controlled in accordance with the drivingcurrent I on the low flow rate side of the operating region. As aresult, fine assisting force control of the power steering system 36becomes possible, thereby obtaining a suitable rigid steering feelingfor maintaining neutral steering position, which leads to an improvementin the steering feeling.

Note that the cross sectional shape of the valve portion 2 a is notlimited to a substantially semi-circular shape as in the embodimentdescribed above. An approximately elliptical shape may also be used, anda cross section having a substantially triangular tapered shape may becombined at its tip end side thereof.

This invention is not limited to the electromagnetic proportional flowrate control valve used for the power steering apparatuses as describedin the above-mentioned embodiments. This invention may also be appliedto an electromagnetic proportional flow rate control valve used for anindustrial machine or the like, and it is clear that variousmodifications are possible within the scope of the technical concept ofthe invention.

Industrial Applicability

This invention can be applied to an electromagnetic proportional flowrate control valve used for power steering apparatuses, industrialmachines, or the like.

1: An electromagnetic proportional flow rate control valve, comprising:a valve body; a valve member including a movable core guided so as toslide freely within the valve body; a spring for driving the valvemember in a valve opening direction; and a solenoid coil for driving thevalve member in a valve closing direction in accordance with an increasein a driving current of the solenoid coil against a spring force of thespring, wherein a solenoid driving force is set such that it is reducedaccompanying a displacement of the valve member in the valve closingdirection with respect to an identical driving current for the solenoidcoil. 2: An electromagnetic proportional flow rate control valveaccording to claim 1, wherein a concave portion for magnetic fieldregulation, depressed into an annular shape, is formed in an innercircumferential surface of the valve body, which structures a magneticcircuit around the movable core. 3: An electromagnetic proportional flowrate control valve according to claim 1, wherein an annular shape gap isformed in a portion of the valve body, which structures the magneticcircuit around the movable core; and a tapered portion for magneticfield regulation is formed opposing the gap. 4: An electromagneticproportional flow rate control valve comprising: a valve body; a valvemember including a movable core guided so as to slide freely within thevalve body; a valve seat that defines a variable orifice with the valvemember; a spring for driving the valve member in a valve openingdirection; and a solenoid coil for driving the valve member in a valveclosing direction in accordance with an increase in a driving current ofthe solenoid coil against a spring force of the spring, wherein aproportion of reduction in an area of an opening of the variable orificewith respect to a stroke of the valve member gradually becomes smalleras the valve member approaches the valve sheet. 5: An electromagneticproportional flow rate control valve according to claim 4, wherein theshape of a tip of the valve member opposing the valve sheet is formedinto a substantially spherical shape. 6: An electromagnetic proportionalflow rate control valve according to claim 2, wherein an annular shapegap is formed in a portion of the valve body, which structures themagnetic circuit around the movable core; and a tapered portion formagnetic field regulation is formed opposing the gap.